FIG. 1 shows a block diagram of the receiver end of a conventional wireless RF system. The RF system comprises an antenna 10 for receiving a weak RF signal which is first amplified by a low noise amplifier 120 in the receiver 12. The amplified RF signal is then mixed in a mixer 124 with the output of a local oscillator 14 to form a mixed signal that includes both intermediate frequency and microwave signals.
A conventional low noise amplifier normally includes two stages in order to achieve the goal of adjusting the power gain of the low noise amplifier. FIG. 2A shows the first stage of a conventional low noise amplifier which comprises a first inductor 20, a second inductor 21, a first MOS transistor 22 and a second MOS transistor 23. The second inductor 21 serves as a serial feedback to match and reduce the noise of the low noise amplifier. The second stage that is not shown in FIG. 2A is used to adjust the power gain of the amplifier.
Because of the two stage circuit design, the conventional low noise amplifier has the drawbacks of high power consumption and complicated design. Furthermore, the second inductor 21 in the conventional amplifier is often designed into the architecture of a single chip. However, this inductance value can not be too small. In order to increase the power gain of the low noise power amplifier, the quality factor Q values of the first and second inductors 20, 21 have to be sufficiently large. Therefore, external inductors with high quality factor Q values are required. This makes the adjustment of the efficiency of the low noise power amplifier very difficult.
FIG. 2B shows the circuit of a conventional Gilbert-type mixer 124. The RF signal is modulated by an oscillation signal in a transconductance stage formed by a third MOS transistor 28 and a fourth MOS transistor 29 to accomplish the effect of wave mixing. The oscillation signal of a local oscillator (LO) enters the mixer 124 through a switching circuit comprising a fifth MOS transistor 24, a sixth MOS transistor 25, a seventh MOS transistor 26 and an eighth MOS transistor 27.
A drawback of the Gilbert-type mixer 124 is that it can not improve the signal to noise ratio and reduce the power consumption at the same time. The current in the switching circuit is proportional to the current in the transconductance stage. The input noise is proportional to the current in the switching circuit but inversely proportional to the current in the transconductance stage. Because of the relationship among the input noise, the current in the switching circuit and the current in the transconductance stage, it is necessary to increase the current in the transductance stage as well as decrease the current in the switching circuit in order to reduce the noise. However, the two requirements are contradictory. The only choice is to increase the power consumption which also increases the noise in the switching circuit. Therefore, a trade-off has to be made between low noise and high power consumption in a conventional mixer. It is very difficult to design a mixer with both low noise and low power consumption.